The invention relates to a method of inhibiting instability during operation of a gas turbine engine, where the instability is due to the uncontrolled interaction between the air filled gap defined by a heat shield and a fuel passage in a conventional fuel injector.
Conventional fuel control systems are designed on the assumption that the fuel is incompressible and flows through a fixed volume conduit system to the injector tips. Therefore fuel control is based on supplying a known volume of incompressible fuel during a known time period.
The inventor has recognized that engine instability at low power levels in particular (known as engine "hooting") is caused by the pressurized fuel interacting with a trapped volume of air in a gap which is conventionally used as an insulator between a fuel injector heat shield and a fuel passage in the fuel injector stem.
The trapped air is compressed and decompressed when fuel pressure changes, and fuel stored in the gap is released in an uncontrolled manner resulting in engine instability.
Conventionally a gas turbine engine includes an elongate fuel injector having an injector stem with an internal fuel passage extending from an engine mount end to an injector tip at a discharge end. The stem includes a tubular internal heat shield disposed within the fuel passage. The heat shield is secured to the fuel passage adjacent the mount end of the stem and spaced inwardly from the fuel passage thus defining an elongate annular thermal insulating gap between the fuel passage and the heat shield.
The air filled gap is open to the fuel passage since it is necessary to permit relative thermally induced movement between the heat shield and the fuel passage. The heat shield is cooled by the flow of relatively cool fuel whereas the fuel injector stem is relatively hot due to the temperature of the surrounding ambient compressed air. To date, the presence of this open air-filled insulating gap has not been considered as problematic, since the assumption has been that coke will quickly form to plug the opening during initial operation. However, it is the timing of coke formation and the unpredictable performance of the coke plug which causes engine instability on initial operation and can result in premature coking of the fuel injector tips.
The air-filled gap causes engine instability since the entrapped insulating air is compressed when pressurised fuel is injected through the fuel passage. The compressed air has less volume and a volume of fuel occupies the area of the air gap from which air has retreated. As a result, the total volume of fuel delivered to the injector tip is less than the volume which the fuel control system records as delivered. When the fuel control reduces fuel pressure, the air within the gap is decompressed and the entrapped fuel within the gap escapes to be delivered to the fuel injectors.
The removal of a volume of fuel when fuel pressure increases and subsequent delivery of fuel when fuel pressure decreases, is the cause of engine instability when such air gaps are used in conjunction with a fuel injector heat shield, especially on initial operation of the engine at low power conditions. After the engine has been in operation for a sufficient time, some of the fuel entrapped within the air gap eventually decomposes due to the temperature of the surrounding fuel stem. Coke deposits form to plug the gap impeding the movement of air and fuel. However, during the initial operation of the engine, the noise and erratic operation of the engine prior to coke formation causes concern to purchasers and the engines are often unnecessarily returned to the manufacturer to investigate the cause of this instability.
The uncontrolled formation of coke and the uncontrolled fuel/air interface within the air gap can cause further fuel system problems. Uncontrolled coke formation within a limited area, combined with the inflow and outflow of fuel within the gap can dislodge coke and cause agglomerations of coke to travel from the gap to the fuel injector tip and spray nozzles. Such movement of coke particles can lead to premature formation of coke in the injector tip and plugging of fuel spray nozzles.
When coke is permitted to form in an uncontrolled and unmeasured manner within the gap, the coke may not adhere firmly to the gap walls or fuel may only partially decompose resulting in undesirable movement of coke particles from the gap to other fuel system components downstream.
The uncontrolled fuel/air interface creates volatile gas within the insulating gap when high engine temperatures cause evaporation of the fuel. The volatile gas may decompose and form coke, however since engine operating temperatures may vary, the ultimate result is unclear. However, the presence of a volatile gas confined in a heated environment is undesirable especially since this gas does nothing to enhance engine performance.
In some situations it is best to merely discontinue use of air-filled insulating gaps in fuel injectors such as in newly manufactured engines. Due to continuing use of such heat shields in existing engines, the disadvantages of use do not overcome the cost of replacement or redesign, and the difficulties described above continue.
It is an object of the invention to prevent engine instability and to control the fuel/air interface where use of air-filled gaps remain.
Further objects of the invention will be apparent from review of the disclosure and description of the invention below.